A filled fluororesin sheet is a sheet of a fluororesin containing a filler. The sheet shows chemical resistance and heat resistance of the fluororesin as well as the inherent functions and properties of the filler. In addition, the filler improves poor creep resistance of the fluororesin. The sheet thus has wide applications including sealing materials.
For use as sealing materials, the properties of fluororesins, for example creep properties, are improved by the addition of fillers such as metals, graphite and inorganic fillers. Such materials are used as gaskets and the like.
Filled fluororesin sheets suited as gasket materials are known in the art. For example, JP-A-2004-323717 (Patent Literature 1) discloses a filled fluororesin sheet suited as a gasket material which contains a swelling mineral and a filler (except swelling minerals).
JP-A-2007-253519 (Patent Literature 2) discloses a process for producing fluororesin sheets which includes a step of rolling a sheet-forming resin composition containing a fluororesin, a filler (such as talc or clay) and a specific solvent under specific conditions. The literature describes that the filled fluororesin sheet obtained by the process achieves high stress relaxation properties and high airtightness and is suited as a gasket material.
Further, fluororesin materials that contain a filler having a specific Mohs hardness are known in the art. For example, JP-B-H06-74810 (Patent Literature 3) discloses a slide member material that contains a polytetrafluoroethylene resin as a main component and 0.5 to 10 wt % of powder of at least one abrasive selected from chromium oxide, silicon carbide, aluminum oxide and mixtures containing at least one of these compounds, the abrasive powder having a Mohs hardness of not less than 5 and an average particle diameter of less than 5 μm. The literature describes that the use of the specific abrasive improves abrasive resistance properties of PTFE.
JP-A-H08-239682 (Patent Literature 4) discloses a slide member composition which is based on a synthetic resin including PTFE and contains 0.003 to 0.3% by volume of inorganic fine particles having a Mohs hardness of not less than 9 and an average particle diameter of not more than 10 μm. The literature describes that the addition in the trace amount of the inorganic fine particles having a high Mohs hardness to various synthetic resin materials realizes synthetic resin slide materials which have excellent abrasion resistance without damaging a mating member or increasing the friction therebetween.
However, gaskets that are composed of the conventional filled fluororesin sheets, for example clay- or talc-filled fluororesin sheets described in Patent Literatures 1 and 2, are still insufficient in terms of stress relaxation properties, in particular at a high temperature.
The present invention has been made in view of the problems in the art described above. It is therefore an object of the invention to provide a filled fluororesin sheet excellent in stress relaxation properties, in particular stress relaxation properties at a high temperature, a process for the production thereof, and a filled fluororesin sheet gasket having excellent stress relaxation properties.
Patent Literatures 3 and 4 do not teach or suggest that the high-hardness filler is added to the fluororesin (such as PTFE) to improve stress relaxation properties of the fluororesin sheet, or the fluororesin containing the high-hardness filler is used as a gasket material.
The present inventor carried out studies and has found that a fluororesin sheet containing a specific filler solves the problems described above. The present invention has been completed based on the finding.